It is quite common in the field where various types of fasteners are utilized in the assembly of materials, that environmental conditions frequently result in rust or corrosion occurring on the fasteners, or between the fasteners and the assembled materials. Many times this may be due in large part to the electrical current that occurs between different materials and especially where the work pieces held together by the fasteners may all be of different materials having different electrical potentials. The rate of decomposition and/or corrosion of the work pieces and the fasteners may also be affected by the nature of the connection of the fasteners and work pieces as a result of the stress level imposed on the connections which may increase the degree of corrosion. External forces applied to the work pieces, or an improper, or interference fit of the fasteners in the work pieces may also affect the corrosion factor.
Heretofore, electroplated metals utilizing cadmium or zinc or any of various organic coatings have been utilized to inhibit corrosion and electrolytic action. However, thin coatings of these materials were relied upon and such coatings were easily ruptured and thus exposed the core metals to the environment. In such circumstances aluminum or plastic coatings would have been more satisfactory since these materials would not readily permit corrosion to occur between the fasteners and work pieces. While such materials may have been superior in many respects to cadmium, zinc and other electroplated materials they did not lend themselves readily to electrodeposition on the fasteners. Further, and more importantly, the degree of corrosion protection afforded by any coating on a fastener of any kind is directly proportional to the thickness and uniformity of the applied coating and since electrodeposition of coating metal is typically limited to thin coatings, fasteners clad in this manner have been lacking in effective coating thickness and totally inadequate from the standpoint of corrosion resistance. In fact, such coatings, or cladding, were quickly destroyed and allowed corrosive reactions to develop between the fasteners and the work pieces.
Other problems incurred by electroplating coatings on fasteners include embrittlement of the coating, as thus applied. Hot dipping of fasteners with zinc or aluminum may provide a coating of adequate thickness, but coatings applied in this manner are frequently lacking in the uniform thickness of the coating necessary to provide the degree of corrosion protection for fasteners thus coated as normally required in the usual operating environment.
Mechanical coating by peen plating has been used to some extent but this type of coating process is restricted to the use of cadmium or zinc and for all practical purposes this prohibits use of this type of process for coating fasteners for the purpose of inhibiting corrosion. Such peen plated parts often are subject to excessive porosity to a prohibitive extent, if applied to fasteners and such that insufficient corrosion inhibiting protection is provided when fasteners coated by this process are utilized in their normal environment.
Fasteners made entirely of aluminum, or other noncorrosive materials have been used in an effort to overcome the problem of excessive corrosion and to some extent this type of fastener may have been successful in meeting some of the problems. However, fastenings of this type have proved lacking in structural properties essential to the proper functioning of the usual fastening installation. These nonferrous fasteners were fabricated from relatively soft material lacking the required strength and rigidity and when hardened became brittle to the point where even in the act of their installation they were subject to fracture. The prior art includes disclosures of the basic concept of extruding a nonferrous coating onto a ferrous metal core. For example Lang et al U.S. Pat. No. 3,399,557 teaches that an aluminum coating can be extruded onto a steel core. This patent however, utilized aluminum billets in the process which precluded continuous extruding which is necessary if a clad product is to be obtained of any length desired. Further, Lange et al relied upon supplemental heating of the cladding metal in order to attain the required temperatures. De Buigne U.S. Pat. No. 3,095,973 and Kaplin U.S. Pat. No. 3,875,782 also teach the extrusion of nonferrous metals around a ferrous metal core but these disclosures also are restricted to processes which are not continuous and this is not inadvertent on the part of the several inventors. Their processes cannot be made to be continuous with the ability to feed billets of infinite length and as a result of this limitation, the processes have found limited acceptability commercially. While Lang et al extrudes the cladding metal around the core metal cross-axially to the direction in which the discrete billets are fed, both De Buigne and Kaplin extrude the cladding metal around the core metal coaxially in the same direction as the feeding of the billets.
Federman U.S. Pat. No. 3,561,399 and Reynolds U.S. Pat. No. 2,543,936 may disclose continuous processes of cladding but in Federman the metal again is fed coaxially and in both of these disclosures the cladding metal enters the cladding die in a molten state. Therefore, in the finished product, the coating comprises a cast structure and as is well known cast structures have physical properties inferior to the properties of wrought structures such as obtained by extrusion.
King U.S. Pat. No. 3,620,119 discloses a coating arrangement for but a portion of an electroplated fastener and is specifically restricted to a particular manner of extruding a "slug" of plastic, or aluminum, onto the fastener.
Harkenrider British Pat. No. 805,617 discloses what he describes as a continuous process of extruding, but he utilizes billets as his feedstock and more importantly the disclosure relates to a linear extrusion method and relies on a heated cylinder and die in plasticizing the billets.
Heretofore, much cladding work has been accomplished by a method of co-extrusion where the core material to be clad was enclosed in the cladding material and the composite structure then extruded through a die orifice. Copper clad aluminum wire was made by this method wherein an aluminum rod was inserted into a copper tube and then this assembly was extruded. In such co-extrusion processes both the core and the cladding material were reduced in cross sectional area during the extrusion operation. Such methods may be acceptable where softer metals such as copper and aluminum may be involved. However, when it becomes necessary to apply a coating such as aluminum, or copper, around a high strength steel core, co-extrusion is difficult, if not impossible, because of the high extrusion pressure which would be required to reduce such a steel core in cross section. Significantly less extrusion pressure would be required if the steel core could be passed through the extrusion die without its cross-sectional area being reduced but reduce only the cross-sectional area of the cladding material. This invention successfully accomplishes that purpose.